worker,supervisor: refactor shutdown signaling mechanics

IO#ready? does not exist in ruby4.

Author: ctrochalakis

lib/rspecq/worker.rb

@@ -87,7 +87,10 @@ def initialize(build_id:, worker_id:, redis_opts:, shutdown_pipe: nil)
     end
 
     def shutdown?
-      @shutdown_pipe&.ready?
+      return false if @shutdown_pipe.nil?
+
+      # returns :wait_readable if the supervisor has not signaled shutdown (closed the writer)
+      @shutdown_pipe.read_nonblock(1, exception: false).nil?
     end
 
     def work